Mineralogical Mapping of Pyroxene and Anorthosite in Dryden Crater Using M³ Hyperspectral Data ^†

Abstract

This study investigates the mineral composition of the lunar Dryden Crater using Moon Mineralogy Mapper (M³) data. A RGB false-color composite reveals distinct pyroxene, anorthosite, and possibly spinel distribution patterns. Orthopyroxenes, excavated from deep crustal layers, dominate steep slopes, while plagioclase-rich materials align with magma ocean models of lunar crustal formation. Minor clinopyroxenes indicate impact melt origins. While space weathering and shock metamorphism pose analytical challenges, integrating spectral data with geological context elucidates the crater’s complex history. The resulting mineral distribution map supports targeted exploration during upcoming lunar missions, resource prospecting and resource utilization initiatives within this geologically complex region.

1. Introduction

The lunar surface displays a relatively limited but geologically significant mineralogical diversity, classically categorized into two primary terrains: the highlands and the maria. The highlands, distinguished by their light-toned appearance, are predominantly composed of anorthosite, which is rich in calcium plagioclase [1,2]. In contrast, the maria are darker basaltic plains formed by effusive volcanic activity and are enriched in mafic minerals such as clinopyroxene (CPX), orthopyroxene (OPX), and olivine [1,3,4]. Remote sensing investigations have demonstrated that the South Pole–Aitken (SPA) basin—within which the Dryden Crater is located—possesses a composition that is notably distinct from the rest of the lunar surface, exhibiting anomalously high concentrations of Fe, Ti, Th, and Mg [5,6,7]. The lithologies exposed in the SPA basin are predominantly ultramafic in character and consist of orthopyroxene, clinopyroxene, olivine, ilmenite, and plagioclase [5,6,7]. These rock types include mare basalts from post-impact volcanism, extensive impact melt deposits, lower crustal fragments, potential upper mantle materials, and ejecta blankets from subsequent impact events following the initial basin-forming collision. Positioned in the inner region of the SPA basin on the Moon’s far side, the Dryden Crater represents a geologically compelling target for high-resolution mineralogical mapping.

2. Materials and Methods

To delineate and characterize the mineralogical distribution within the Dryden Crater, we employed L2 M3G20090814T061444_V01_RFL hyperspectral image cube data with spectral resolution of 85 channels acquired by Moon Mineralogy Mapper (M³), available at the Planetary Data System (PDS) of National Aeronautics and Space Administration (NASA) https://pds-imaging.jpl.nasa.gov/volumes/m3.html (accessed on 11 December 2024). We incorporated topographic data obtained by the merging of Lunar Orbiter Laser Altimeter (LOLA) and the Kaguya TC Digital Elevation Model (DEM) “Moon LRO LOLA—SELENE Kaguya TC DEM Merge 60N60S 59 m v1”, “SLDEM2015” [8]. Thanks to SLDEM2015, we georeferenced the M³ image in Environment for Visualizing Imagery (ENVI 5.6). Then, we utilized a false color composite (FCC) approach, adopted from [1,3,4], to generate RGB FCC mineral image-map of the Dryden lunar impact crater in Equirectangular map projection [9]. The resulting map presents the spatial distribution of CPX, OPX minerals, and anorthosite, key components of the lunar crust, in 150 m spatial resolution. To derive the map, we integrated three spectral parameters derived especially for utilizing M³ data. We calculated the pyroxene ratio (Px) [3], the anorthosite ratio (An) [3] and the spinel parameter (Sp) [4] by applying the ENVI formulas, see

Table 1. Employed spectral indexes on M3 data.

ENVI Formula	General Formula	Spectral Index Name
(b7 + b32)/b19	(R700 nm + R1200 nm)/R950 nm	Pyroxene Ratio (Px)
(b22 + b47)/b34	(R1000 nm + R1500 nm)/R1250 nm	Anorthosite Ratio (An)
(((b34 − b9)/(500)) × 1350 + b34)/b76	(((R1250 − R750)/500) × 1350 + R1250)/R2600	Spinel Parameter (Sp)

. Following the above calculations a Red, Green, Blue (RGB) FCC was generated by assigning the parametric image of Px to the red channel, Sp to the green channel, and An to the blue channel [4]; see

Table 2. Employed RGB false color composite after [4].

RGB False Color Composite	General Formula
PxSpAn	Red = Pyroxene ratio Green = Spinel parameter Blue = Anorthosite ratio

. This multispectral band substitution technique enhances the mineralogical discrimination and spatial visualization of lithological units.

3. Results

The analysis of the RGB composite map of pyroxenes and anorthosite in the Dryden Crater shown on Figure 1 reveals distinct color variations corresponding to different mineral compositions:

• Red Pixels—Indicate strong absorption features at 1000 nm and 2000 nm, characteristic of high-Ca, Fe-rich CPX [10,11,12]. These are associated with impact melt sheets and are observed on the slopes of small craters within Dryden, suggesting local impact melt formation.
• Yellow Pixels—Show absorption features at shorter wavelengths (~900 nm and ~1800 nm), indicative of low-Ca, Mg-OPX [6,10,11,12]. These are found on steep slopes of the central peak, crater walls, and surrounding hummocky terrain, suggesting excavation of deep crustal materials [13] or an impact melt origin in noritic lithologies [7].
• Blue Pixels—Display a lack of absorption at 1000 nm and 2000 nm, but exhibit an absorption feature at 1250 nm, indicating the presence of pure anorthosite (PAN) [2,14,15]. However, given their location in shadowed regions and the potential for masking by mineral mixing and shock metamorphism [16], these are interpreted as potential false positives for PAN.
• Green-Cyan Pixels—Exhibit a lack of strong absorption at 1000 nm and absorption features at 1250 nm and 2000 nm, suggesting plagioclase feldspar mixtures potentially containing small amounts (<~5 vol%) of spinel within a feldspathic matrix [14,17,18,19]. These are extensively distributed within the crater and throughout the surrounding ejecta.

4. Discussion

The red pixels in the composite are indicative of clinopyroxene-dominated lithologies, such as impact melt sheets or basaltic terrains. The absence of volcanic activity within the Dryden Crater depression, combined with the specific distribution of red pixels on the slope of a small impact crater on the crater floor, suggests that these features may represent impact-excavated melt sheet overlaid by low-shock materials deposited subsequently. Furthermore, the presence of red pixels within small craters located on the crater wall, terrace and crater ejecta implies that the impact melt could have formed during the formation of these smaller craters rather than solely being associated with the larger Dryden Crater.

The yellow pixels indicate material consistent with Mg-OPX [6], which may have originated either from deep within the lunar crust or the upper mantle as a result of impact excavation or impact melt processes. Mg-OPX, specifically high-magnesian enstatite, is typically associated with noritic lithologies. Rocks from the Dryden Crater containing enstatite are generally of deep origin, potentially forming distinct plutons or representing a significant component of the lower crust. The transport of these deep-seated materials to the lunar surface likely occurred during impact events associated with the heavy meteorite bombardment. Consequently, yellow pixels mark regions dominated by norites [7] or other Mg-OPX-rich materials. Our analysis reveals that Mg-OPX minerals are exposed on the steep slopes of the central peak, crater walls, fresh small craters, and the slopes of the hummocky terrain surrounding the crater floor. These locations exhibit fresh geological features that are less affected by space weathering, indicating a notable presence of Mg-OPX in the material composition of the Dryden Crater. Furthermore, the central peaks of impact craters bring to the surface materials originating from deep beneath the crater floor, making them the deepest exposed materials within the crater, indicating the presence of Mg-OPX at Dryden’s subsurface depths which overlaps the inner ring of the Apollo basin in the northwestern side and exposes a large noritic province encompassing the basin itself [7].

Plagioclase feldspar, particularly anorthosite (composed of ≥90% calcium-rich plagioclase feldspar), is essential for understanding lunar crustal evolution. Although PAN typically exhibits a diagnostic absorption feature near 1250 nm, in mineral mixtures, this feature can be weak and easily masked by more strongly absorbing minerals like pyroxenes. Furthermore, shock metamorphism—a common process on the lunar surface—can transform crystalline plagioclase into diaplectic glass [16], effectively erasing the 1250 nm absorption feature even in the absence of other minerals. While the direct spectral identification of anorthosite is often hindered by mineral mixing and shock metamorphism, considering the regional geological context and the lunar magma ocean (LMO) hypothesis, anorthosite remains a significant component of the overall soil composition [20].

Spinel exhibits a broad and deep composite absorption band spanning ~1250–3500 nm. These spectral characteristics are influenced by variations in the mineral’s Mg, Fe, and Cr content, as described in [17]. Although small amounts of spinel can be challenging to distinguish, they contribute to the overall spectral features of the surface material mixture. The green-cyan indications are extensively distributed throughout the Dryden Crater’s depression and the surrounding area. This interpretation is consistent with a weak 1000 nm feature and a deep 2000 nm feature [14,18,19]. It also aligns with the geological processes that have shaped the Dryden Crater and its surroundings, including lunar crust formation, local geological activity, and impact events.

5. Conclusions

This study integrated hyperspectral data from the Moon Mineralogy Mapper with geological context to characterize the mineral composition of the Dryden Crater on the lunar far side. Our analysis reveals a complex lithological mosaic shaped by early crustal differentiation and subsequent impact events. The abundance of OPX within Dryden, particularly on steep slopes, suggests excavation from deep within the lunar crust or upper mantle. This unroofing could be linked to large-scale events such as the SPA basin-forming impact, the subsequent Apollo basin impact, or the Dryden-forming event itself. The presence of plagioclase feldspar-rich materials (anorthosite) is consistent with models of early lunar crustal differentiation involving plagioclase flotation in a magma ocean [20]. Furthermore, the identification of spinel-bearing lithologies supports the role of complex magmatic and impact processes in shaping the lunar surface [14,18,19]. Minor occurrences of CPX likely originate from impact-excavated melt sheet overlaid by low-shock materials at the crater floor and from impact melt sheets generated by small cratering events within and around Dryden. It is crucial to acknowledge the limitations inherent in remote sensing. Factors such as space weathering and shock metamorphism can alter spectral signatures, potentially obscuring diagnostic features and requiring careful interpretation to accurately identify surface compositions [16,21]. As demonstrated in the detection of anorthosite, careful analysis and geological context should be considered while addressing such detections. Despite these challenges, this study demonstrates the utility of combining hyperspectral data with geological context to characterize the mineral composition of a lunar crater. By integrating these spectral and geological datasets, this study advances our understanding of the Moon’s crustal evolution, highlighting the interplay of early differentiation and later impact-driven modification. The resulting pyroxene and anorthosite distribution map of the Dryden Crater provides insights into its geological history. By identifying distinct mineralogical assemblages and their spatial distribution, this research supports targeted exploration during upcoming lunar missions, resource prospecting strategies, and resource utilization initiatives within this geologically complex region. Furthermore, the methodology employed has cross-planetary applicability for remote sensing investigations of other airless bodies.